1. Field of the Invention
This invention relates to a regenerative node for a communications network and particularly, but not exclusively, to ring networks for two way video signal communication between pairs of nodes.
2. Related Prior Art
A number of local area network (LAN) topologies and access protocols have been proposed or developed which meet the requirements for data or voice transmission. Optical fibres have been recognised as a means of providing very wide bandwidths for LANs with prospects of interactive video services. One of the problems with existing LAN designs is that they do not lend themselves to easy upgrading either from data to telephony or from telephony to video transmission. For the full bandwidth potential of optical fibres to be exploited, consideration of the limitations of the network topologies and access protocols is required at the outset and, if possible, an evolutionary programme defined so that installations can be upgraded without loss of service to initial users. A further factor which must be considered is the initial cost of provision of the network. It is unrealistic to add a surcharge for future services to a network which initially has only a rudimentary function.
One of the first LAN structures to emerge was Ethernet. This has a bus topology and transmits data packets via a contention access protocol. Optical fibre derivatives of this system have been proposed based upon a star topology. Ethernet is, however, unsuitable for broad band transmission because the collision probability would be too high. The number of collisions increases as the data rate, packet size or network size increases.
A ring structure is better suited to high data rate transmission. Each node on the ring is functionally similar to a repeater in a binary transmission link where the upper speed is limited by the technology used. Two access protocols for such systems are the Cambridge ring and the Orwell ring (Adams, J. L., and Falconer, R. M., "ORWELL: A protocol for carrying integrated services on a digital communications ring", Electronics Letters, Vol. 20, No. 23, p 970, 8 Nov. 1984). Both systems use packet transmission and are therefore limited in speed by the processing and storage electronics which are required to assemble the packets.
To overcome the speed limitation of these protocols and other similar ones based upon packet transmission, a number of hybrid protocols have been proposed. These allow part of the time window to be used for data packets whilst part of the window is reserved for a time division multiplex (TDM). TDM allows data to be transmitted in regular time-slots without the need for packet formation and storage. TDM is particularly suited to transmission of services such as speech or video, where a guaranteed delay time is required.
Broadcast quality video transmission requires a data rate approximately 1000 times that required for speech. The high data rates associated with video transmission, and the numbers of channels required for a useful number of users, give rise to serial transmission rates approaching the state of the art for electronic circuits.
One method of the TDM approach is discussed in an article entitled "A time division multiplex approach to high data rate optical network design" by David W. Faulkner (Proc Fibre Optics '87, SPIE Vol. 734, ppl-6). The system described suffered from two design limitations. The first was the use of an inefficient line code (CMI) which allowed only 8 channels even though the system bandwidth would allow twice that number. The second was the need for digital electronics in the customer demultiplexer which operated at the multiplex rate. Both of these limitations have been overcome using channel scramblers prior to multiplexing and channel selection using a sampler and delay-lock loop in the receiver demultiplexer as described in detail in the applicant's co-pending patent application GB 8804552 (U.S. application Ser. No. 07/348,575 filed Apr. 28, 1989). The receiver there described selects a single channel by sampling the input multiplex at the channel rate with a clock phase determined by a delay-lock loop. A channel is selected by setting a locally generated descrambling sequence at a slight frequency offset from the incoming channels. Delay-lock occurs when this sequence is in bit-synchronism with an incoming channel sequence, producing descrambled data of a suitable form. Such a receiver is hereinafter referred to as "delay-lock receiver". When the data is a video signal, the deterministic component can be the line blanking interval which is detected and used to control the loop. The delay-lock receiver locks onto the TDM channel scrambled by the same sequence locally generated by the receiver, the receiver sequence. A particular TDM channel can therefore be selectively directed to chosen delay-lock receivers by scrambling the data with the receiver sequence of that receiver or receivers.